1. Field of the Invention
The present invention has to do with polyurethane wear layers for floor and wall coverings wherein the specular gloss of the polyurethane wear layer is determined by controlling the thermal conditions of a substrate, a liquid polyurethane coating composition before it is coated on the substrate and/or the coated substrate before the polyurethane is cured. More specifically, we have found that controlled changes in the thermal conditions of the 1) substrate prior to applying the liquid polyurethane coating composition, 2) the liquid coating composition and 3) the substrate after applying the liquid coating composition but before curing, provides different and predictable specular gloss indices without otherwise changing the curing conditions or the composition of the polyurethane. The polyurethane compositions employed according to the invention must contain matting agents.
2. The Related Art
Wear layer surfaces of floor covering products, such as resilient sheet materials, tiles and wood, are usually prepared from plastisols or polyurethanes, or a layer of plastisol and a layer of polyurethane, or other combinations of layers as are known to those skilled in the art. Most plastisols for vinyl flooring products are formulated using polyvinylchloride (PVC) emulsion resins mixed with primary and secondary plasticizers, extenders, stabilizers and other additives. Plastisol wear layers are discussed in detail in U.S. Pat. No. 5,679,721 and polyurethane wear layers are discussed in U.S. Pat. No. 5,169,704.
It is known to modify the gloss of a surface layer by adjusting the atmospheric conditions of curing. For example, a method of producing flat (non-glossy) films is described in U.S. Pat. No. 3,918,393, assigned to PPG Industries, Inc., wherein a radiation sensitive or actinic light sensitive composition is cured under ionizing radiation or actinic light and two curing stages are employed. The first stage comprises partial curing in air and the second stage comprises completing curing in an inert gas atmosphere, for example, a nitrogen atmosphere.
According to U.S. Pat. No. 4,169,167, the gloss of energy-curable coating and ink compositions is reduced by exposing such compositions to actinic radiation in an oxygen rich atmosphere at different intensity levels. The curable polyurethanes employed according to the patent must include a flatting agent in order to attain any effective degree of gloss reduction.
Low gloss and gloss controlled radiation cured coatings are obtained by employing different radiation wavelengths at different curing stages according to U.S. Pat. No. 4,313,969. Coating compositions containing inert particles are employed and gloss control is obtained by adjusting the spectral distribution, the intensity, or the dose of the initial radiation, or by adjusting the time interval between the initial and subsequent irradiation steps. Another reference which describes the use of different wavelengths to control surface properties of a surface coating is U.S. Pat. No. 4,411,931 wherein surface texture is controlled with a three stage UV curing process employing various wavelengths and curing atmospheres.
Matting agents are employed according to the present invention and they are incorporated in liquid polyurethane compositions. The matting agents are comprised of microparticles which are made of mineral and/or polymeric materials and the microparticles are incorporated in the liquid. Polymeric microparticles and dispersions of polymeric microparticles in coating compositions are discussed in U.S. Pat. No. 4,569,966. The microparticles can act as a processing aid and they can affect the characteristics of the coating compositions before and after they are cured.
We have now discovered that the specular gloss of a polyurethane wear layer can be controlled by manipulating the thermal conditions of 1) a substrate, 2) a liquid polyurethane coating composition before it is coated on the substrate and/or 3) a coated substrate prior to curing the polyurethane, wherein the liquid polyurethane coating composition contains a matting agent. We have found that variations in the temperature of any one of the three elements referenced above has an impact on curing speed and viscosity of the liquid coating which determines the location of the matting agent relative to the surface in the cured product and accordingly affects specular gloss of the product. Higher temperatures cause the viscosity to drop and this helps the matting agent to migrate to the surface, especially when the microparticles of the matting agent have a low density relative to the liquid polyurethane. However, higher temperatures above a certain threshold (which varies with the polyurethane chemistry) also cause faster curing which slows migration of the microparticles. These competing factors are controlled according to the present invention to permit manufacturers to control the specular gloss of the finished coating using a low cost temperature control technique.
We have also discovered a method of predicting the gloss characteristics based upon the viscosity and chemical reactivity of a polyurethane photopolymer and the energy supplied during curing.
Floor covering and wall covering products having a polyurethane wear layer are prepared using a polyurethane which has matting agents incorporated therein. A photo-curable or radiation-curable liquid polyurethane incorporating matting agents is conventionally coated over a substrate which comprises a surface covering material requiring a wear layer or an additional wear layer of polyurethane. The temperature of the substrate prior to coating the polyurethane thereon is adjusted to a predetermined value, the temperature of the liquid polyurethane is adjusted to a predetermined value prior to coating it on the substrate and the temperature of the substrate after the polyurethane is coated thereon is adjusted to a predetermined value, whereby the combination of substrate temperatures before and after coating the polyurethane thereon, and the coating temperature before coating it on the substrate, will cause the subsequently cured polyurethane to have a pre-determined specular gloss. Changes in any one or more of these temperatures will change the specular gloss of the cured polyurethane wear layer. No changes in the curing atmosphere (e.g., air, nitrogen) or the energy of the curing atmosphere or the composition of the polyurethane are required to obtain the changes in specular gloss which are brought about by the process of the invention. Accordingly, the invention provides a technique for producing products with different specular gloss characteristics on the same production line at little or no cost and without varying the formulation of the liquid polyurethane coating.